They're all essentially four (well really more like 3 1/2) versions of the same game; which version do you prefer?
For me, it's a tough call. The NES game obviously has the most nostalgia for me (it was my first CV, back in 1989) and set the stage for every ClassicVania to come; Vampire Killer was (for me) a mish-mash of Castlevania and Simon's Quest (which I also love); X68K took a lot of my favorite parts of Castlevania (right down to a few of the stages themselves) and made them prettier and with better music; and Chronicles had improved versions of a few of my favorite songs from X68K and Hairband Simon. Overall, though, I'd have to say the NES game, because it brings me the most nostalgia and is the one I'm most consistently able to play all the way through without getting tired of playing it.
And yes, I know Castlevania IV is technically yet another remake, but it's not nearly as similar to the original as the others I listed. (And besides that, if IV were included, the question itself would be completely needless.)